German Patent No. 40 03 227 discloses a fuel injection valve with a tubular core surrounded by a magnet coil. An armature which is connected to a valve closing body that interacts with a valve seat is connected to the spray end of the core. The valve seat is formed on a valve seat body which is held in a tubular valve seat carrier. The valve seat body serves at the same time as a component for the magnetic reflux of the magnetic flux circuit. For this purpose, the valve seat carrier surrounds the armature like a sleeve on the end opposite the spray end and guides the magnetic reflux radially to a guide element which connects the valve seat carrier to the core axially and thus closes the magnetic flux circuit. To prevent a magnetic short circuit between the core and the valve seat carrier serving as reflux body, the valve seat carrier is isolated from the core by a nonmagnetic intermediate part designed as a metallic valve inside tube. Therefore, the magnetic flux does not travel directly from the core into the valve seat carrier, but instead it intentionally follows an indirect path through the armature, so an axial force component is exerted on the armature to open the fuel injection valve when the magnet coil is electrically energized.
However, the design of the metallic intermediate part as an additional turned part requires an additional manufacturing expense and an additional assembly expense. The core, the valve seat carrier and the nonmagnetic intermediate part are manufactured as turned parts and must be joined by two welds. The welds also serve to provide the hydraulic seal, so that fuel is prevented from entering the annular space holding the magnet coil. Therefore, special demands must be made of the quality of the welds, and a simplified welding method such as a spot welding method cannot be used. Furthermore, eddy currents which would have a negative effect on the efficiency of the electromagnetic operation of the valve can occur in the nonmagnetic metallic intermediate part.
German Patent No. 195 03 821 has already proposed that the core and the valve seat carrier be designed as a one-piece part. The magnetic isolation between the core and the valve seat carrier is achieved by a magnetic throttle point. The one-piece combined component forming the core and the valve seat carrier is designed with an extremely small wall thickness of 0.2 mm, for example, in the area of the throttle point. When the magnet coil is magnetically energized, a saturation flux density is achieved in this area, so that the magnetic flux is throttled, and the magnetic flux exceeding the saturation flux density of the throttle point travels from the core to the armature and into the valve seat carrier. Due to the small wall thickness in the area of the magnetic throttle point, however, the mechanical stability of the combined part forming the core and the valve seat carrier is impaired, so that assembly requires great caution and precision. The saturation flux density occurring at the throttle point is essentially unavailable for the magnetic operation of the armature and thus it has a negative effect on the efficiency of the electromagnetic operation. The current flux in the magnet coil must be increased accordingly, and the resulting thermal power loss must also be dissipated.